Technological Field
The present invention relates to an image forming apparatus.
Description of the Related Art
An image forming apparatus such as a printer, a copier, and a combined machine includes a sheet table (e.g., a tray or a cassette), in which a plurality of sheets to be used as a recording medium for an image is to be set. The image forming apparatus performs printing by conveying a sheet from the sheet table to a printing position in the apparatus.
A function of setting an operation condition in accordance with the type of the sheet to obtain an appropriate image is known as a function of the image forming apparatus of this type. For example, in an electrophotographic image forming apparatus, a sheet is classified by basis weight, and, for example, conveyance velocity (process velocity), development bias, transfer bias, and fixing temperature are set in accordance with the basis weight. The setting can prevent, for example, jams, development defects, transfer defects, and fixing defects.
Methods in which the image forming apparatus acquires the type of the sheet include a method in which a user selects and specifies the type of the sheet from several options (e.g., plain paper, thick paper 1, and thick paper 2). The image forming apparatus sets an operation condition in accordance with the type specified by the user.
Unfortunately, users have difficulty in correctly specifying the type of a sheet since the types of sheets usable in an image forming apparatus have recently been diversified. For this reason, attention is paid to a method in which the image forming apparatus automatically detects the type of a sheet based on output from a predetermined sensor.
A configuration in which a so-called media sensor for detecting the type of the sheet is disposed on a conveyance path is known. According to the configuration, the type can be determined by detecting physical quantity such as translucency and thickness, which are difficult to be detected in the state where sheets are stacked on a sheet table. In addition, in an apparatus including a plurality of sheet tables, one media sensor can detect the type of a sheet regardless of from which sheet table the sheet is ejected.
When the media sensor is disposed on the conveyance path for the sheet, preparation (finishing) for image formation is performed in parallel to conveyance of the sheet to a sensor position in order to shorten first print output time (FPOT) during execution of a print job. The preparation for image formation in an electrophotographic image forming apparatus includes processing of, for example, rotating a photoconductor to be charged.
In the preparation for image formation, the operation condition corresponding to one of a plurality of types assumed for a sheet is determined as a provisional condition. For example, the rotation velocity of a photoconductor and charging bias are set as forming an image under the provisional condition. When the detected type of the sheet is different from the type corresponding to the provisional type, the provisional condition is switched to a fixed condition corresponding to the detected type detected. After image formation under the fixed condition is made possible, the image formation is started.
JP 2013-019946 A discloses a traditional art for reducing delay of the start of image formation in the case where an operation condition is switched after preparation for the image formation is started. In JP 2013-019946 A, the peripheral velocities of a photoconductor drum and an intermediate transfer belt are changed with toner interposed in a nip portion between the photoconductor drum and the intermediate transfer belt, in an image forming apparatus in which a drive source of the photoconductor drum is different from the drive source of the intermediate transfer belt.
According to the technique of JP 2013-019946 A, even when peripheral velocities of a photoconductor drum and an intermediate transfer belt are deviated from each other owing to different drive sources during switching from a provisional condition to a fixed condition, wear on the photoconductor drum and the intermediate transfer belt is reduced by interposing toner.
Toner of coloring material, however, is required to be kept attached to a photoconductor with a developing device turned on over a period in which rotations of the photoconductor drum and the intermediate transfer belt are stabled and the peripheral velocities thereof are equalized at least immediately before and after switching of the operation condition. Unfortunately, the coloring material is wastefully consumed.